This invention relates to a fuel cell system, and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power for vehicle propulsion.
Fuel cells have been used as a power source in many applications. Fuel cells have also been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and air is supplied as the oxidant to the cathode. PEM fuel cells include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distribution the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack, typically adjacent cells, is referred to as a cluster.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies which comprise the catalyzed electrodes, are relatively expensive to manufacture and require certain controlled conditions in order to prevent degradation thereto.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to this reaction: CH3OH+H2Oxe2x86x92CO2+3H2. The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,442 and 08/980,087, filed in the name of William Pettit in November, 1997, and U.S. Ser. No. 09/187,125, filed Nov. 5, 1998, and each assigned to the assignee of the present invention. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, each assigned to the assignee of the present invention.
For vehicular power plants, the reaction within the fuel cell must be carried out under conditions which preserve the integrity of the cell and its valuable polymeric and precious metal catalyst components. Since the anode, cathode and electrolyte layers of the MEA assembly are each formed of polymers, it is evident that such polymers may be softened, melted or degraded if exposed to too high a temperature.
A typical fuel cell system does not directly monitor the rate of hydrogen flow to the fuel cell; that is, a hydrogen sensor is not located directly upstream of the fuel cell. In such a fuel cell system, it is important to match the load being demanded of the fuel cell with the rate at which hydrogen is supplied to the fuel cell. If more current is attempted to be drawn out of the fuel cell than the fuel cell is capable of supplying because there is not enough hydrogen to create the increased electrical power, then it is possible to significantly degrade the fuel cell stack.
If the vehicle propulsion system continues to increase the load and allows the cell voltage to continue to decline, deterioration of one or more individual cells can result and it is also possible to incur a permanent reverse polarity. In this situation, a cell begins acting as a resistor and will start heating up. As the cell continues to heat up, it will adversely affect the cell next to it and, if heat effect is not abated, it is possible to melt components of the fuel cell.
Although it is possible to obtain the overall voltage of the fuel cell stack, this does not indicate the existence of one problem cell within the stack. In other words, a small voltage drop occurring at a number of the cells could not be distinguished from a large voltage drop in one problem or weak cell.
This is evident by an example where the fuel cell stack might have, for example, 200 cells at 0.7 to 0.8 volts each at a given load. In a circumstance where 3 cells drop from 0.75 volts to 0.0 volts the overall fuel stack voltage changes from 150 volts to 147.75 volts. This latter value is well above the expected voltage if all of the cells were at 0.7 volts, that is, at the lower range indicating a stack voltage of 140 volts which is nominally acceptable.
While it would be advantageous to monitor the voltage of each cell in a stack, from an economic view point this is not strictly necessary or desirable. Since a typical PEM fuel cell stack, sized for use in automotive power and voltage ranges, contains approximately 150 to 200 cells, the logistic of reading all of the 150 to 200 cell voltages can become a significant task, with respect to hardware connections. Also, due to the sheer size of the data being processed from each of the 150 to 200 cells, care must be taken to design efficient software to collect and process all of the cell voltage information.
Therefore, a typical approach relies on monitoring groups of cells, referred to as xe2x80x9cclustersxe2x80x9d instead each individual cell. Care must be taken not to group too many cells together in a cluster since the total contribution of each cell""s output to the chosen cluster output must be large enough so that an individual poor performing cell can be resolved from the condition where are cells in the cluster are on the low side of nominal performance. This resolution limitation usually results in either three or four cells being grouped together in a cluster.
Historically, when forcing a cluster monitoring requirement on the fuel cell stack design, the result is that the stack must be designed with a total number of cells which have a numerical modulus equal to the number of cells in each monitored cluster in order not to leave any cell(s) unmonitored. This poses a problem when there is a need to change the number of cells in a cluster for a given stack design. Additionally, adding xe2x80x9cextraxe2x80x9d cells to a new fuel cell design just to make the number of cells convenient for the monitoring system is not considered a good economic practice.
Thus, it would be desirable to provide a method for diagnostic monitoring of fuel cell clusters containing any number of cells per cluster which can be interfaced to fuel cell stack designs containing any total number of cells. It would also be desirable to provide a method for diagnostic monitoring of a fuel cell stack which avoids the design and calibration of a unique monitoring approach only for a cluster containing less than the identical number of cells per cluster in the remaining portion of the fuel cell stack. It would also be desirable to provide a monitoring technique for a fuel cell which enables a fixed number of cells to be included in each cluster without requiring rearrangement or rewriting of the monitoring software for the addition or removal of cells from the fuel cell stack.
The present invention is a method of monitoring a fuel cell apparatus wherein a hydrogen-rich stream is supplied to a fuel cell stack, wherein the fuel cell stack includes a plurality of first clusters formed of identical number of fuel cells and at least one second cluster formed of a different number of fuel cells. The present method includes the following steps:
(a) measuring the voltage across the entire fuel cell stack;
(b) measuring the voltage across the second cluster of fuel cells; and
(c) creating a pseudo cluster voltage for the second cluster based on the average cell voltage of all of the cells in the fuel cell stack, the actual voltage across the second cluster and the difference between the number of cells in the first clusters and the number of cells in the second cluster.
In a specific aspect, the method of the present invention includes the following steps:
(a) measuring the total voltage across all of the fuel cells in the stack;
(b) determining the average cell voltage for the entire stack;
(c) multiplying the average cell voltage for the entire stack by the difference between the number of cells in each of the first clusters and the number of cells in the second cluster; and
(d) adding the product of (c) to the actual voltage across the second cluster.
In another specific aspect, the inventive method determines the difference between the number of cells in each of the first clusters and the number of cells in the second cluster to form a constant;
multiplies the constant and the average cell voltage across the entire stack; and
adds the product of step (b) to the actual measured voltage across the second cluster.
In a broad aspect of the invention, the present inventive method adjusts the measured voltage across the second cluster by a factor equivalent to the average cell voltage of all the cells in the fuel cell stack for each different fuel cell between the number of fuel cells in the second cluster and the number of fuel cells in the first cluster.
The method of the present invention provides significant advantages in fuel cell design and diagnostic monitoring. The present inventive method enables the cells of a fuel cell stack to be monitored in clusters containing a small number of cells regardless of the number of cells forming each cluster. This enables a single monitoring program or circuit to monitor each fuel cell cluster regardless of the number of cells in each cluster. This avoids the necessity of designing a new monitoring control program or circuit just for the clusters containing a different number of cells. More importantly, the monitoring method of the present invention enables a fuel cell stack to be devised with any number of cells to meet particular application requirements without the necessity of adding or eliminating individual fuel cells just for monitoring purposes.